SDS gel electrophoresis of microtubule proteins obtained from bovine brain by polymerization cycles revealed a new protein of 18 kDa. This protein was copolymerized with tubulin and its stoichiometry to tubulin remained constant for at least 5 cycles of assembly. Moreover, this protein remained bound to microtubules stabilized with 10 μM taxol and pelleted through a 4 M glycerol cushion. The same 18 kDa protein was found in a purified preparation of the high molecular mass microtubule‐associated protein 1 (MAP‐1). The 18 kDa protein copurified with the MAP‐1 heavy chains during column chromatography on phosphocellulose, DEAE‐cellulose, hydroxyapatite and Bio‐Gel A‐15m. Incubation of the MAP‐1 preparation with a mouse monoclonal antibody to the light chain 1 (LC‐1) of MAP‐1 and with a second precipitating antibody (a rabbit antibody to mouse IgG) immunoprecipitated from the solution all the known components of MAP‐1 (heavy chains, LC‐1, LC‐2), as well as the 18 kDa protein. Immunoblotting showed, however, that this antibody does not interact directly with the 18 kDa protein. These results indicate that the 18 kDa protein forms a complex with all other components of MAP‐1. This polypeptide, therefore, is a new light chain (LC‐3) of M AP‐1.